X-ray CT apparatuses are used to irradiate with X-rays a subject when the periphery of a subject who is lying on the side is irradiated on the table top of a bed. Because the intensity of the X-rays transmitted through the subject is detected with an X-ray detector, projection data can be collected on the horizontal profile of the subject. After the table top of the bed has then been moved, the next projection data can be collected in the same manner for the next horizontal profile (transverse cross-section), so that the projection data can be collected repeatedly by moving the table top until multiple transverse cross-sections of projection data of the subject have been collected by repeating the table top moving and projection data collecting operations. The projection data corresponding to multiple transverse cross-sections of a subject is then processed with a high-speed data processing apparatus and image reconstruction processing is conducted to obtain image data containing multiple transverse cross-sections of a given patient.
When X-ray CT apparatuses are used to obtain scanned data, projection data is collected in multiple transverse cross-sections with an X-ray CT device, image data is created for multiple cross-section when image reconstruction processing is applied to the projection data of multiple transverse cross-section with a high-speed data processing device and the image data of these multiple transverse cross-sections is displayed with a display device, so that a laser image or the like can be created of the images of these multiple transverse cross-sections, or burnt on a film which can be observed. An abnormal region of a subject can thus be observed with these operations which are widely used. This made it possible to realize a method wherein image data ranging from several pages to hundreds of pages can be recreated initially with an X-ray CT device.
According to recent methods used for scanning of data with and X-ray CT device, projection data of multiple transverse cross-section is collected with an X-ray CT device, image reconstruction processing is applied to the projection data of these multiple transverse cross-sections with a high-speed data processing device, and image data corresponding to multiple transverse cross-section is created, and this image data of multiple transverse cross-sections is stored in electromagnetic disk devices, optomagnetic disk devices, optical disk devices, and other types of magnetic media. The image data of the multiple transverse cross-sections stored in this magnetic media is then read so that the image data can be displayed on a display device and observed. In addition, when the image data of the multiple transverse cross-sections, which has been stored in this magnetic media, is read, image processing can be applied to these images in an image processing device. For example, a three-dimensional construction can be created from this image data with a three-dimensional image display device so that three-dimensional images can then be observed.
With the initial X-ray devices, a subject was lying on the side and irradiated with x-rays of a moving table top, enabling reciprocal collection of the projection data of the subject in this manner. However, as the technology used in X-ray CT devices underwent a rapid progress, projection data was collected with the latest X-ray CT devices, so that the subject was lying on the side and X-ray irradiation was performed simultaneously with the movements of the table top. This method for collection of projection data is called helical scanning.
Because projection data was collected with the initial X-ray devices when a subject was lying on the side and X-ray irradiation was applied together with the movements of the table top for reciprocal collection of the projection data, the transverse cross-section of the collected projection data was created as a transverse cross-section of image data obtained with reconstructed image data. Specifically, since the transverse cross-section of the image data obtained with the reconstructed image data corresponded to the transverse cross-section of the collected data, the number of the transverse cross-sections of the collected image data was identical to the number of the pages of the image data obtained when image reconstruction operations were performed. In addition, the positions of the transverse cross-sections containing collected projection data were distributed in a dispersed fashion in the body axial direction of the patient. Incidentally, when projection data is collected while simultaneous X-ray irradiation operations are conducted with the helical scanning method, transverse cross-sections of dispersed collected projection data are not present. Because of that, virtual transverse cross-sections are set so that interpolation processing is applied to the projection data to determine projection data corresponding to the virtual transverse cross-sections, and the projection data of these virtual transverse-cross-sections is used when image reconstruction operations are performed.
Because virtual transverse cross-sections of this projection data can be set with a considerable amount of freedom, this also makes it possible to select, with a considerable amount of freedom, the number of pages of the image data that can be obtained from the image reconstruction processing with a sequential setting of virtual cross-sections of the projection data, and also to select the interval between the transverse cross-sections.
In one example of the initial X-ray CT devices, X-ray detectors where used to detect the intensity of X-rays transmitted through the patient in the body axial direction of the patient, and the projection data was collected in one transverse cross-section of the patient. Due to the rapid progress of the technology used in X-ray CT devices, X-ray detectors are arranged in the latest X-ray CT devices in multiple arrays in the body axial direction of the subject and multiple arrays of X-ray detectors are used for simultaneous collection of the projection data in multiple transverse cross-sections of the subject. When these multiple arrays of detectors are used for simultaneous scanning with the helical scanning method, much more projection data can thus be collected than when scanning was conducted with the initial X-ray CT devices. When these multiple detectors are used with the helical scanning method, since transverse cross-sections of the collected projection data will not be present in a dispersed fashion as was the case with the initial X-ray CT devices, virtual transverse cross-section are generally set so that projection data is determined for virtual transverse cross-sections, while interpolation processing is applied to the projection data and image reconstruction operations are conducted by using the projection data of these virtual transverse cross-sections. Because virtual transverse cross-sections of this projection data can be set with a considerable amount of freedom, this makes it possible to select freely the number of pages of the image data obtained with the image reconstruction processing with the consequent setting of virtual transverse cross-sections of projection data, as well as to select the interval between the transverse cross-sections.
During scanning using an X-ray CT device, currently, projection data is collected with X-ray CT scanning devices and image reconstruction processing operations are applied to this projection data with high-speed data processing devices. The result of this image reproduction processing is that multiple obtained transverse cross-section of image data are stored on a storage medium and preserved in magnetic disk devices, optomagnetic disk devices, optical disk devices and the like. The methods used with these results of detection of the X-ray CT devices include burning of images created by using the image data of obtained multiple transverse cross-sections as a result of image reconstruction processing, methods used to observe such a film, observation methods which are used when images are displayed in image observation devices using the image data of multiple transverse cross-sections, image processing involving operations such as three-dimensional image processing performed with image processing devices and applied to the image data of multiple transverse cross-sections obtained as a result of image reconstruction processing, methods used to observe these results, etc.
Although each of these usage methods requires parameters optimized for image reconstruction processing, currently, the data that is stored for a long period of time is image data available after image reconstruction, and since long-term storage of projection data prior to the image reconstruction has not been performed because a major computer resource is required for image reconstruction of projection data, there has been no other research of reconstruction processing using optimized image reconstruction processing parameters with each of the respective usage methods.
In particular, in cases when the structure of three-dimensional processing is created by using the image data of multiple transverse cross-sections, image reconstruction processing parameters suitable for direct observation of transverse cross-section images can be increasingly obtained with results enabling to use image data of multiple transverse cross-sections of reconstructed images with image reconstruction processing parameters that have been optimized for various types of three-dimensional image processing operations. However, because image data is generally preserved for a long period of time only after image reconstruction, and because long-term storage of the projection data has not been provided, it was thus not possible to examine other objectives of image reconstruction processing which are used for each individual operation.
Also, even when long-term storage has been provided, for example of the projection data of X-ray CT devices, since continuous scanning operations with the X-ray CT devices are conducted as a routine with a plurality of scans, it is impossible for an external user to perform image reconstruction by using desirable image reconstruction parameters, which an external user will need for this projection data. Therefore, the image reconstruction of the projection data has been conducted with image reconstruction parameters determined in advance with a routine scan.
There are also types of device enabling to perform high-speed image processing, such as three-dimensional image processing which can be applied to image data obtained with image reconstruction using the latest X-ray CT devices. However, even in a system which is equipped with this function, since continuous scanning operations are conducted routinely with many scans, it is still in fact impossible for an external user to perform three-dimensional image processing with this function.
Because unlike in the initial CT devices, the noise level of the X-ray scanners is decreased while the spatial density of the X-ray scanners is increased in latest multi-array scanners and CT devices which use the helical scan method, and also the pitch density of helical scan is increased, a very precise spatial distribution of the collected projection data is created. Because of that, a very precise image can be obtained with a small increase of the noise level also when the spatial region in which image reconstruction is performed is reduced, while the number of the image elements participating in image reconstruction is preserved. Accordingly, the spatial precision of the images in the region of interest in a given subject can be increased by decreasing the image reconstruction region for the same image projection data. In addition, since a meaningful image can be obtained with only a small increase of the noise level also with a narrow interval between the image reconstruction screens, this also makes it possible to increase the spatial resolution in the body axial direction.
Because a precise spatial distribution of projection data collected with the latest multi-array scanners and CT devices using the helical method has been achieved, CT values (i.e., intensities of the centers of voxels) can now be determined in spatial positions with a small interval not only inside the transverse cross-section of a patient, but also in the body axial direction. In three-dimensional image display devices which use CT image data, voxel data is created from the image data of transverse cross-sections obtained during image reconstruction of projection data accumulated in the body axial direction, and a three-dimensional image is displayed by applying three-dimensional image reconstruction operations to this data. When CT image data obtained with the latest design of multi-array detectors and CT devices using the helical scan method is used, this makes it possible to achieve a very high precision of three-dimensional images.
To create three-dimensional images from CT image data, image reconstruction of projection data obtained with X-ray CT devices is performed, the image data of the transverse cross-sections is created, and voxel data is created when this image data is stacked up in the body axial direction. The image elements in the transverse cross-section, for example 512×512 pixels when an image measurement methods using 0.5 mm ×0.5 mm are applied to image data in the body axial direction, for example with an interval of 0.5 mm, creating a stack of 512 sheets, will contain a spatial region corresponding to 256×256×256 mm and a stereoscopic voxel image will be created with 512×512×512 individual voxel data elements.
Next, when this stereoscopic voxel image is processed with three-dimensional reconstruction processing using surface rendering and volume rendering, a three dimensional image can be created and displayed.
Three-dimensional display devices that have been used up until now perform image reconstruction of projection data obtained with X-ray CT devices and create CT image data. The structure of voxel data is created by using this data. When images containing two-dimensional images of CT images are interpreted by a doctor who is a radiology specialist, a series of CT images displaying a transverse cross-section of a patient is arranged in a plane and the doctor is interpreting the image by observing the image and looking for the presence or absence of abnormalities. On the other hand, to interpret a three-dimensional image, the structure of the voxel data is created from this series of CT image data, a degree of opacity is added to the voxel structure based on the voxel physical properties, that is to say based on the CT values of this voxel data, a light source is applied in the direction of the visual line of an observers who is observing this voxel data and when light emitted from this light source passes through an object, and an image is created with integration of the calculations obtained from attenuation and reflection of this data. Accordingly, because the calculation of integration corresponding to the voxel number of the displayed images will thus be required, the higher the voxel number, the longer the calculation time that will be needed for volume rendering. Specifically, because according to the volume rendering method, each CT value is allocated to the opacity corresponding to light permeation characteristics, and control is maintained over all the voxel values by using light attenuation along the direction of the line of sight based on the variable density calculated with the grey-level gradient method and applied in all the volume points, the brightness value is calculated by multiplying the amount of incident light from a light source by the opacity of the voxel structure. A three-dimensional image is then obtained with sequential integration (recasting) of this data in the direction of the line of sight. Because natural and smooth variations can thus be obtained with the volume rendering method even in edges created by rapid fluctuations of the CT value, this makes it possible to improve dramatically the drawing function which is used to draw fine and detailed tissues, such as the peripheral tissues of blood vessels. Although the surface rendering method represented the main trend of methods used for three-dimensional displaying in the past, the volume rendering method has been used increasingly at present.
With similar three-dimensional display devices used up until now, projection data was collected when a subject was scanned with an X-ray CT device and multiple sheets of CT image data were used with the reconstructed image data containing this projection data. Although the density resolution was often more important than factors such as spatial resolution when CT images were observed as two-dimensional images, the spatial resolution is in some cases more important than the density resolution when three-dimensional images created from CT image data are observed, and when two dimensional images are created from two-dimensional image data, different parameters are often optimal to create the image data structure, when compared to cases when three-dimensional images are observed. However, the current situation is such that CT image data is used for reconstruction with image reconstruction parameters optimized for observation of two-dimensional images containing projection data with X-ray CT devices, and three-dimensional image reconstruction is performed by using CT image data optimized for two-dimensional image observation with three-dimensional image display devices. This is because a major computer resource is required for reconstruction of images from projection data and also a major storage capacity is needed to store accumulated data, etc.
The following points are worth mentioning with respect to X-ray CT devices:    1) X-ray radiation is emitted with the fan shape along the inner plane of a transverse cross-section from a source of X-ray radiation, enabling operations along the outer periphery of the transverse cross-section of a subject, and X-rays transmitted through the subject are measured for example with a detector which measures 500 items. Therefore, for example 500 items can be measured in one position of the radiation source. Projection data is collected when repeated operations are applied to cover 180 degrees of the outer periphery of a transverse cross-section in this manner. For example, since data is collected in more than 180 directions for each repeated operation, projection data corresponding to 500×180=90,000 will be collected.    2) Convolution processing is conducted after preprocessing and the like has been performed to eliminate noise from the projection data.    3) Data collected in a position corresponding to the transverse cross-section of a subject, for example when a flat surface is set for image elements corresponding to 512×512 pixels creating a construction of image elements having a rectangular shape covering 1×1 mm, and reversed projection is created with a fan shape for projection data, after convolution processing has been conducted from the position of the source of X-rays, when projection data is collected for each image element on the flat surface containing the image elements.    4) At this point, because data creating a reversed projection will not necessarily be cut laterally in the center of the picture elements of a flat surface of a picture containing the picture elements, interpolation processing is conducted in the vicinity of the reversed projection data and similar data is allocated to each picture elements.    5) When this reversed projection processing is applied repeatedly to all of the collected projection data, each picture element of a picture element flat surface corresponding to 512 picture elements×512 picture elements is used for reconstruction of image data, having a value which corresponds to the physical properties obtained with X-ray irradiation of a subject.    6) If the position of the transverse cross-section of a subject is moved, for example by 1 mm in the body axial direction, the image data of the transverse cross-section is reconstructed by collecting projection data in the same manner and applying image reconstruction processing operations to this data.    7) When these operations are repeated if the position of the transverse cross-section of a subject has been moved in the body direction, image data of transverse cross-sections of a subject corresponding to 500 sheets can be collected for example at an interval of 1 mm. When sets of this data are used, this makes it possible to create a voxel space corresponding to 512×512×500 voxels, creating for example the construction of stereoscopic image elements (voxels) of 1 mm×1 mm×1 mm.    8) Three-dimensional images can be created and displayed when three-dimensional image processing is applied with volume rendering and the like to this voxel space created in this manner.
FIG. 1 is a block diagram showing a simulation of a three-dimensional display device connected to a network with a conventional X-ray CT device. Element 101 is an X-ray CT device, 111 is a scanner part of a CT device, 112 is a data collection part, displayed as a simulation model in the figure. When detection is performed with an X-ray scanning device so that a subject is scanned with a scanner part using X-rays from a source of X-ray radiation, a digitized system of the collected projection data is created in the data collection part. This digitized output data 122 of the X-ray detector is sent to a preprocessing part 113 and operations during which noise is eliminated from the data, correction is applied, etc., are conducted in the part. The projection data 123 preprocessed in this preprocessing part 113 is then sent to an image reconstruction device 114.
Image reconstruction processing is then applied in the image reconstruction device 114 to the projection data 123 once preprocessing operations have been finished. The image data 124 processed with image reconstruction processing is sent to a console part 115 of the X-ray CT device, the data is displayed and at the same time also stored in image data storage device 116. Reference numeral 221 indicates image data which is transmitted to a external three-dimensional image display device or the like.
Reference numeral 201 designates a three-dimensional image display device, 211 is an image data storage device which stores image data transmitted from the X-ray CT device, and 212 is a three-dimensional image processing device. The three-dimensional image processing device is used so that when an operator specifies image data 222, signal is received from the image data storage device 211 and three-dimensional image reconstruction operations involving volume rendering and the like are applied to this signal, and the created three-dimensional image 223 is displayed on a console 213.
As was shown in this example, with conventional three-dimensional image display devices, image data processing in CT applications starts with acquiring projection data (the “raw” data set) from a CT scanning device, then reconstructing sliced digitized image data (a voxel data set) based on the raw data, and then rendering three-dimensional images on a computer display screen based on the voxel data set. The rendering processing is always based on the voxel data set which is reconstructed once and stored on disk or other storage device. In other words, the rendering processing did not utilize the projection data set, because reconstruction was very time consuming, so it was not practical to use the projection data for rendering.
During scanning operations using an X-ray CT device, projection data is collected with an X-ray CT device, image reconstruction processing is applied with a high-speed data processing device to this projection data, and image data comprising multiple transverse cross-sections obtained as a result of this processing is stored in a magnetic disk device, electromagnetic disk device, optical disk device or the like. While suitable parameters exist for image reconstruction processing according to respective methods using results that have been detected with an X-ray CT device, prior to the present invention, since long-term storage is not provided for projection data if image data exists for image data that has been stored after image reconstruction processing operations, image reconstruction processing has not been realized with parameters that have been optimized for image reconstruction processing according to each respective method.
In particular, when the construction of three-dimensional data is created by using the image data of multiple transverse cross-sections, the image reconstruction processing parameters that are compatible with direct observation of images of transverse cross-sections are different from image reconstruction processing parameters that are compatible with processing of three-dimensional images in an increasing number of cases, when it would be desirable to used the image data obtained during image reconstruction. However, image reconstruction processing is generally not conducted for this specific purpose because a major storage capacity is needed in order to accumulate projection data. Further, when long-term storage of projection data is not performed, since most of the time is spent on routine processing performed by image reconstruction devices of X-ray CT devices, the development is not open to users outside of the system. Therefore, the image reconstruction devices thus cannot be used even if reconstruction of projection data is desirable for three-dimensional images because a major computer resource is needed for reconstruction of images using projection data.